Large core holey fibers

ABSTRACT

Various types of holey fiber provide optical propagation. In various embodiments, for example, a large core holey fiber comprises a cladding region formed by large holes arranged in few layers. The number of layers or rows of holes about the large core can be used to coarse tune the leakage losses of the fundamental and higher modes of a signal, thereby allowing the non-fundamental modes to be substantially eliminated by leakage over a given length of fiber. Fine tuning of leakage losses can be performed by adjusting the hole dimension and/or the hole spacing to yield a desired operation with a desired leakage loss of the fundamental mode. Resulting holely fibers have a large hole dimension and spacing, and thus a large core, when compared to traditional fibers and conventional fibers that propagate a single mode. Other loss mechanisms, such as bend loss and modal spacing can be utilized for selected modes of operation of holey fibers. Other embodiments are also provided.

PRIORITY APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/844,943 filed May 13, 2004 entitled “LARGE CORE HOLE FIBERS”(Attorney Docket IMRAA.024A) which claims the benefit of U.S.provisional application 60/536,914 filed Jan. 16, 2004 entitled “LARGECORE HOLEY FIBERS” (Attorney Docket IMRAA.024PR) both of which areincorporated herein by reference in its entirety.

BACKGROUND

1. Field

The present teachings relate to optical fibers in general and inparticular, to single-mode and multi-mode holey fibers having large coredimensions and devices and systems that use such fibers such as lasersand amplifiers.

2. Description of the Related Art

A holey fiber is an optical fiber where optical guidance is provided byan arrangement of “holes” disposed about a core. Such holes may have arefractive index lower than that of the core, and collectively form acladding region having an effective refractive index also lower thanthat of the core.

More generally, holey fiber is used herein to refer to fiber comprisinga plurality of cladding features, e.g., holes, disposed in a matrix,wherein the cladding features may have a refractive index different fromthat of the matrix. Accordingly, light incident on the cladding featuresmay be reflected or scatter therefrom.

A photonic crystal fiber (PCF) is a holey fiber where the holes arearranged in a regular manner. In one arrangement, the cladding regioncan be formed by holes arranged in a hexagonal close-packing-likemanner, and the core can be formed in the center area of such hexagonalpatterned cladding region. The core can be in the center area where acenter hole, and one or more inner “rings” of holes, would otherwise belocated in the extended array of holes. PCFs formed in the foregoingmanner thus have a core dimension and a cladding region thickness.

As with traditional fibers, sustained propagation of only a single modein PCFs is desirable in many applications. Conventional PCFs operable inthe single-mode propagation regime have core dimensions and claddingregion thicknesses that resemble traditional non-holey single-modefibers. One limitation of such single-mode traditional fibers andconventional PCFs is that the core dimension is limited to a relativelysmall value. In such small core fibers, high optical power levels resultin peak intensities that induce non-linearities in the properties of thecore and fiber. High power application of such fibers, therefore, iseither not practical or not possible.

SUMMARY

In certain embodiments described herein, large core fibers forpropagating single optical modes can be used in lasers and amplifiersand for other systems. One embodiment of the invention, for example,comprises an optical fiber for propagating a fundamental mode andsuppressing higher order modes. The optical fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d, said plurality of        cladding features being substantially arranged in a plurality of        rows, N; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        fundamental mode to said core region, said plurality of cladding        features having (i) sufficient average spacing, Λ, to provide an        increased effective core size, (ii) a sufficient average size,        d, to provide substantial confinement of light within said core        region, and (iii) an average number of rows, N, suitable to        support propagation of said fundamental mode and suppress        propagation of said plurality of higher order modes,    -   wherein said average size, d, and said average spacing, Λ, have        respective values that yield a ratio of d/Λ that is at least        about 0.35 to provide an increased effective core size and        substantial confinement, and said average number of rows N is        less than or equal to about 4 so as to provide single mode        propagation.

Another embodiment of the invention comprises an optical fiber forpropagating a single optical mode. The optical fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        single optical mode to said core region, said plurality of        cladding features having (i) sufficient average spacing, Λ, to        provide an increased effective core size, 2ρ, and (ii) a        sufficient average size, d, to provide substantial confinement        of light within said core region,    -   wherein said average size, d, and said average spacing, Λ, have        values at least about 20 micrometers to provide an effective        core size of at least about 20 micrometers.

Another embodiment of the invention comprises an optical fiber forpropagating a fundamental mode and suppressing higher order modes. Theoptical fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d, said plurality of        cladding features arranged over a portion of said matrix having        an average thickness, t; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        fundamental mode to said core region, said plurality of cladding        features having (i) sufficient average spacing, Λ, to provide an        increased effective core size and (ii) sufficient average size,        d, to provide substantial confinement of light within said core        region, and (iii) said portion of said matrix including said        cladding features having suitable average thickness, t, to        support propagation of said fundamental mode and suppress        propagation of said plurality of higher order modes,    -   wherein said average size, d, and said average spacing, Λ, have        respective values that yield a ratio of d/Λ that is at least        about 0.4 so as to provide an increased effective core and        substantial confinement, and said average thickness is less than        or equal to about d−3Λ so as to provide single mode propagation.

Another embodiment of the invention comprises an optical fiber forpropagating a single optical mode. The optical fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        single optical mode to said core region, said plurality of        cladding features having (i) sufficient average spacing, Λ, to        provide an increased effective core size, 2ρ, and (ii) a        sufficient average size, d, to provide substantial confinement        of light within said core region,    -   wherein said average size, d, and said average spacing, Λ, have        respective values that provide an effective core size of at        least about 40 micrometers.

Another embodiment of the invention comprises an optical fiber forpropagating a single optical mode. The optical fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        single optical mode to said core region, said plurality of        cladding features having (i) sufficient average spacing, Λ, to        provide an increased effective core size, 2ρ, and (ii) a        sufficient average size, d, to provide substantial confinement        of light within said core region,    -   wherein said average size, d, is substantially equal to said        average spacing, Λ.

Another embodiment of the invention comprises an optical fiber forpropagating a single mode. The optical fiber comprising:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d; and    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        single mode to said core region, said plurality of cladding        features having (i) sufficient average spacing, Λ, to provide an        increased effective core size, 2ρ, and (ii) a sufficient average        size, d, to provide substantial confinement of light within said        core region,    -   wherein the difference between said average size, d, and said        average spacing, Λ, is less than about 50% of said average        spacing, Λ.

Another embodiment of the invention comprises an optical fiber assemblycomprising an optical fiber that supports a fundamental mode having afundamental mode profile and a plurality of higher order modes havingdifferent mode profiles. The optical fiber assembly comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix;    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of        light to said core region, said core region being sufficiently        large so as to support propagation of said fundamental optical        mode and said plurality of higher order modes; and    -   an optical coupler disposed with respect to said core region to        couple light into said core region, said optical coupler        configured to output light having an optical distribution        substantially matching the mode profile of said fundamental        mode, thereby coupling substantially more of said light into        said signal mode of said fiber than said plurality of higher        order modes.

Another embodiment of the invention comprises a method of propagatinglight through an optical fiber that supports a fundamental optical modeand a plurality of higher order modes. The fundamental mode has a modeprofile with a spatial distribution. The method comprises:

-   -   coupling light into a core region of an optical fiber, said core        region surrounded by a cladding region comprising a plurality of        cladding features disposed in a matrix; and    -   propagating said light along said core region, substantially        confining said propagation of said light along said core region        with said plurality of cladding features forming said cladding        region,    -   wherein substantially more of said light is coupled into said        fundamental mode of said fiber than said plurality of higher        order modes.

Another embodiment of the invention comprises a hollow core photonicband gap fiber that provides optical amplification. The hollow corephotonic band gap fiber comprises:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix;    -   a hollow core region surrounded by said cladding region, said        hollow core having a hollow region formed therein, said        plurality of cladding features arranged to form a photonic band        gap to substantially confine propagation of light in said hollow        core region; and    -   a gain region having optical gain that is disposed proximal to        said hollow region such that said light propagating through said        hollow core region is amplified by said gain provided by said        gain region.

Another embodiment of the invention comprises an optical fibercomprising:

-   -   a cladding region comprising a plurality of cladding features        disposed in a matrix, said plurality of cladding features having        an average spacing, Λ, and an average size, d, said plurality of        cladding features being substantially arranged in a plurality of        rows, N;    -   a core region surrounded by said cladding region, said plurality        of cladding features substantially confining propagation of said        light in said core region, said plurality of cladding features        having an average spacing, Λ, and a average size, d, and an        average number of rows to support propagation of a fundamental        mode and a plurality of higher order modes; and    -   at least one mode filter that suppresses higher order modes by        introducing substantially greater loss to said higher order        modes than to said fundamental modes such said fundamental mode        dominates while said higher order modes are quenched.

The optical fiber described herein can be used for many applications.The optical fiber may be used for example as gain fiber and may beemployed in lasers and amplifiers including fiber lasers and fiberamplifiers. Other uses are possible.

For example, another embodiment of the invention comprises an opticalfiber amplifier comprising:

-   -   an optical gain fiber for propagating a single mode, said        optical gain fiber providing optical gain, said optical gain        fiber comprising:    -   (i) a cladding region comprising a plurality of cladding        features disposed in a matrix, said plurality of cladding        features having an average spacing, Λ, and an average size, d;        and    -   (ii) a core region surrounded by said cladding region, said        plurality of cladding features substantially confining        propagation of said single mode to said core region, said        plurality of cladding features having (i) sufficient average        spacing, Λ, to provide an increased effective core size, 2ρ,        and (ii) a sufficient average size, d, to provide substantial        confinement of light within said core region,    -   wherein the difference between said average size, d, and said        average spacing, Λ, is less than about 50% of said average        spacing, Λ.

Another embodiment of the invention comprises an optical fiber amplifiercomprising:

-   -   an optical pump source; and    -   an optical gain fiber optical fiber for propagating a single        optical mode, said optical gain fiber providing optical gain,        said optical gain fiber comprising:    -   (i) a cladding region comprising a plurality of cladding        features disposed in a matrix, said plurality of cladding        features having an average spacing, Λ, and an average size, d;        and    -   (ii) a core region surrounded by said cladding region, said        plurality of cladding features substantially confining        propagation of said single optical mode to said core region,        said plurality of cladding features having (i) sufficient        average spacing, Λ, to provide an increased effective core size,        2ρ, and (ii) a sufficient average size, d, to provide        substantial confinement of light within said core region,    -   wherein said average size, d, is substantially equal to said        average spacing, Λ.

Another embodiment of the invention comprises a CW fiber lasercomprising:

-   -   a resonant cavity formed by at least partially reflective        surfaces; and    -   an optical fiber in said resonant cavity, said optical fiber        having optical gain, said optical fiber propagating a single        optical mode, said optical fiber comprising:    -   (i) a cladding region comprising a plurality of cladding        features disposed in a matrix, said plurality of cladding        features having an average spacing, Λ, and an average size, d;        and    -   (ii) a core region surrounded by said cladding region, said        plurality of cladding features substantially confining        propagation of said single optical mode to said core region,        said plurality of cladding features having (i) sufficient        average spacing, Λ, to provide an increased effective core size,        2ρ, and (ii) a sufficient average size, d, to provide        substantial confinement of light within said core region,    -   wherein said average size, d, and said average spacing, Λ, have        values at least about 20 micrometers to provide an effective        core size of at least about 20 micrometers.

Other embodiments are possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and B schematically illustrate an exemplary holey fiber havinga cladding region defined by a plurality of “holes” or cladding featuresthat facilitates propagation of only a single optical mode through arelatively large core;

FIGS. 2A and B schematically illustrate one embodiment of a photoniccrystal fiber (PCF) having a plurality of holes or cladding featuresarranged in a two-dimensional representation of an exemplary simplecubic array, wherein the dimension of the holes, spacing between theholes, number of layers or rows of holes, and/or composition of theholes can be varied to allow propagation of only a single optical modein a relatively large core;

FIGS. 3A and B schematically illustrate one embodiment of a PCF having aplurality of holes or cladding features arranged in a two-dimensionalrepresentation of an exemplary hexagonally close-packed array, whereinthe dimension of the holes, spacing between the holes, number of layersor rows of holes, and/or composition of the holes can be varied to allowpropagation of a single optical mode through a relatively large core;

FIGS. 4A-C schematically illustrate various embodiments of thehexagonally close-packed array of FIGS. 3A and B, showing differentnumbers of “rings” (i.e., rows or layers) of holes/cladding featuresdistributed in an array about a relatively large core, wherein thenumber of rings can be used to affect the leakage losses of variousoptical modes of an optical signal;

FIGS. 5A-C schematically illustrate how the hole dimension can be variedto affect the leakage losses of various modes;

FIGS. 6A-C schematically illustrate how the hole spacing can be variedto affect the leakage losses of various modes;

FIG. 7 schematically illustrates one embodiment of a PCF having arelatively large core capable of supporting propagation of only a singleoptical mode, and further comprising an outer cladding region formed bya plurality of holes for supporting multiple modes;

FIG. 8 schematically illustrates one embodiment of a PCF having arelatively large doped core region capable of single mode propagation;

FIGS. 9A-F schematically illustrate various embodiments of holey fiberhaving asymmetric hole arrangements disposed about a relatively largecore so as to facilitate polarization maintaining (PM) single-modepropagation;

FIGS. 10A-D schematically illustrate how an effective core dimension ofa PCF can be changed by “removing” different number of inner “rings” orlayers, wherein such core dimension can be used to allow propagation ofselected modes through a relatively large core;

FIG. 11 schematically illustrates one embodiment of a PCF having a dopedring or annular section disposed about a hollow region the core toprovide gain;

FIG. 12 illustrates loss as a function of number of layers forfundamental and first order modes;

FIG. 13 illustrates loss as a function of inverse bending radius forfundamental and first order modes;

FIG. 14 illustrates effective modal index as a function of inversebending radius for fundamental and first order modes;

FIG. 15 schematically illustrates two exemplary holes or claddingfeatures of a PCF, where a quantity d represents each hole's diameterand Λ represents the spacing between the holes;

FIGS. 16A-C are schematic representations illustrating the operatingcharacteristics of a PCF, where single-mode and multi-mode propagationregimes are separated by curves representative of different numbers oflayers on a plot of λ/Λ versus d/Λ;

FIG. 17 schematically illustrates two exemplary hole features havingdifferent shapes and dimensions; and

FIGS. 18A and B are schematic representations illustrating operatingcharacteristics of a holey fiber, where single-mode and multi-moderegimes are separated by curves representative of different effectivethicknesses for cladding regions on a plot of λ/Λ_(avg) versusd_(avg)/Λ_(avg).

DETAILED DESCRIPTION OF SOME EMBODIMENTS

These and other aspects, advantages, and novel features of the presentteachings are apparent from the following detailed description and theaccompanying drawings. In the drawings, similar elements have similarreference numerals.

The present teachings generally relate to holey fibers, photonic crystalfibers, and photonic bandgap fiber. For the purpose of descriptionherein, and in accordance with current usage of terms of art, a holeyfiber is an optical fiber where optical guidance is essentially providedby a regularly or irregularly arranged cladding features or “holes.” Asused herein, holes refers to features disposed in a matrix materialhaving different optical properties such as index of refraction incomparison to the matrix material. The holes can be open, evacuated, orfilled with air, gas, liquid, or may comprise other material,transparent or non-transparent, such as for example glass, polymer, orother substance or material. A photonic crystal fiber (PCF) is a holeyfiber with substantially regular arrangement holes. A photonic bandgapfiber (PBF) is a PCF with appropriate spacing and arrangement of theholes or cladding features that satisfies the Bragg condition at thewavelength of operation, and essentially provides optical guidance byphotonic bandgap effects.

FIG. 1A shows a segment of a holey fiber 100, while FIG. 1Bschematically illustrates an exemplary cross-section of the holey fiber100 across the line 1B-1B. As the cross-section shows, the holey fiber100 comprises a cladding region 102 that can be defined by a ring-likeor annular region having an average inner radius ρ and an average outerradius R. The cladding region 102 comprises a plurality of holes orcladding features 110.

The inner radius ρ further defines a core 106 of the holey fiber 100.The core 106 may or may not be formed from same material as that of anouter region 112 and/or spaces between the holes 110 (the matrix inwhich the holes are formed). In various embodiments, the core 106,spaces between the holes 110, and the outer region 112 are formed fromglass having a first refractive index n₁.

In certain embodiments, the holes 110 can be formed from materialshaving a second refractive index n₂ less than n₁, including but notlimited to vacuum, a gas, a mixture of gases, a second type of glass, orother solid or liquid material. (By contrast, various embodiments ofphotonic band gap fiber may comprise holes or cladding features 110 thatare formed from materials having a second refractive index n₂ greaterthan the index of refraction of the core 106 or surrounding matrix.) Asdepicted in FIG. 1B, the holes 110 may have different dimensions(exemplified by a dimension d_(i)). Furthermore, spacings between theholes 110 may be different (exemplified by a spacing Λ_(j)). Moreover,although the holes 110 are depicted as being round, other non-roundholes may also be used.

One aspect of the present teachings relates to selectively manipulatinga parameter or some combination of parameters such as the claddingregion's effective thickness (R-ρ), average hole size (d), average holespacing (Λ), and hole composition. As described above, the holes may beformed from different material. For the purpose of portions of thisdescription, it will be assumed that the holes have a refractive indexthat is less than that of the core although the holes need not be solimited and may have a greater index of refraction as described above.By manipulating the foregoing properties of a holey fiber, one canselectively control the leakage losses of the different modes of anoptical signal propagating through the core. For example, in variousembodiments, one mode is propagated with a substantially smaller leakageloss while substantially all other modes propagate with substantiallylarger leakage losses.

As used herein, single mode and multimode fiber are defined consistentlywith the definitions used for traditional non-holey fiber. Fortraditional fibers, single mode and multimode fiber are generallydefined in terms of V number, which is equal to π (numerical aperture)(core diameter)/wavelength for step index fibers. For non-step indexfibers, numerical aperture and core diameter can be calculated with theequivalent values for step index fibers [see, e.g., Martinez, F., Husey,C. D., “(E)ESI determination from mode-field diameter and refractiveindex profile measurements on single-mode fibres” IEEE Proceedings V135, pp. 202-210, (1988)]. For fibers satisfying the relationship V<2.4,the power of the fundamental mode is significantly larger than theoptical power of the next higher mode. Alternatively, for fibers whereinV>2.4, at least the next mode above the fundamental mode can havesignificant power in comparison to the fundamental mode. Single mode andmultimode traditional fibers are accordingly specifically defined by therelationships V<2.4 and V>2.4, respectively. V=2.4 is the cut-off forthe propagation of any mode but the lowest order mode.

In holey fibers, the numerical aperture can be found by the differencein refractive index of core and cladding. However, a core diameter thatis the equivalent value for step index fibers is difficult to calculate.Various references [see, e.g., (1) Knight et al, “Properties of photoniccrystal fiber and the effective index model” J. Opt. Soc. Am. A Vo. 15,pp. 748-752, (1998), and (2) Mortensen et al “Modal cutoff and the Vparameter in photonic crystal fibers” Opt. Lett. V. 28, pp. 1879-1881,(2003)] report that if the core diameter is made equal to the pitch orthe distance between holes, Λ, then the V for cut off for thepropagation of any mode other than the single mode is 2.5 (see, e.g.,Knight et al) and π (see, e.g., Mortensen et al). For the variousembodiments described herein, whether the V cut-off is 2.405, 2.5 or πis not critical. Various embodiments of holey fiber described hereinhave a much larger core radius than possible with conventional opticalfiber that supports propagation of a single optical mode. Therefore, wewill utilize the recent research in this technical area where multimodefiber is defined as where V>π and the core diameter is made equal to thepitch or average pitch to the fiber. Conversely, single mode fiber isdefined herein as fiber where V<π.

As described above, holey fiber may be designed to introduce loss forspecific modes. The hole size, spacing, and the number of holes may, forexample, be selected to induce loss in the propagation of higher ordermodes in a multimode fiber where V>π. With a decrease of the number ofholes, light in the higher order modes may not be confined to the coreand may escape from the fiber. Such loss introduced into multimode fiber(V>π) is analogous to traditional non-holey multimode fiber having a Vnumber larger than π that include mode filtering provided, for example,by bending the fiber to introduce loss in the propagation of higherorder modes. (Mode filters are described in, e.g., U.S. Pat. No.5,818,630 issued to Fermann et al on Oct. 6, 1998 and entitled“Single-mode Amplifier and Compressors Based on Multi-mode Fibers,”which is hereby incorporated herein by reference.) Sufficient bendingcan be applied to induce losses for each of the modes higher than thefundamental mode such that the fundamental mode is the sole mode thatpropagates through the bent multimode fiber. Similarly, multimode holeyfiber having a V number larger than about π may have a design thatintroduces loss to higher order modes so that propagation of thesehigher order modes is attenuated.

FIGS. 2A and B, and subsequent figures depict various holey fibershaving regularly spaced holes in various configurations. Thus, theseholey fibers are now referred to as PCFs (photonic crystal fibers) inaccordance with the definition set forth above. It will be understoodthat unless stated otherwise, concepts disclosed in reference to variousPCFs are also applicable to aperiodic structure such as for example theexemplary holey fiber described above in reference to FIGS. 1A and B aswell as asymmetric and other structures having varying degrees of order.Furthermore, the various geometric arrangements of holes in the PCFsherein are exemplary, and should not be construed to limit the scope ofthe present teachings as a wide range of variations in configurations,arrangements, and feature characteristics are possible.

FIGS. 2A and B illustrate an exemplary PCF 120 having a plurality ofholes 130 arranged in a two-dimensional representation of a simple cubicarray. A core 126 may be defined by a region devoid of holes that iscreated by the absence of a center hole (removed in FIG. 2B) in thesimple cubic array. Accordingly, a measure of the core size is 2ρ, whichcorresponds to the diameter of the central hole absent from the array.An exemplary one-layer cladding region 122 can be located between, e.g.,an inner ring 124 a (radius ρ) and an outer ring 124 b (radius R) thatsurrounds the first layer of holes 130. More layer(s) of holes can bearranged beyond the first layer shown in FIG. 2B. Furthermore, a corecan be altered by removing more than the center hole (in which case,number of rows or layers in the array would otherwise be greater thanone).

The exemplary cladding region 122 provides the core 126 whose diameter2ρ is comparable to hole dimension d and hole spacing Λ. By selecting arelatively large hole dimension, and thereby a relatively hole spacing,the corresponding core diameter can be relatively large. Propagation ofonly a single optical mode can be achieved by selectively adjusting thevalues of d, Λ, and/or the number of layers.

FIGS. 3A and B illustrate an exemplary PCF 140 having a plurality ofholes 150 arranged in a two-dimensional representation of an exemplaryhexagonal close-packed type array. A core 146 may comprise a region thatoverlaps with an absent center hole removed in FIG. 3B and from thehexagonal array. A first layer 142 comprises six holes 150 arranged in ahexagonal manner, and defines the core 146 having a radius ρ. Asdescribed below, more layer(s) or rows of holes can be arranged beyondthe first layer 142 shown in FIG. 3B. Furthermore, a core can be definedby removing more than the center hole (in which case, the number oflayers or rows in the array would otherwise be greater than one).

The exemplary cladding region 142 provides the core 146 whose diameter2ρ is generally comparable to hole dimension d and hole spacing Λ. Byselecting a relatively large hole dimension, and thereby a relativelylarge hole spacing, the corresponding core diameter can be relativelylarge. Single-mode propagation, propagation of only a single opticalmode to the exclusion of sustained propagation of higher order modes,can be achieved by selectively adjusting the values of d, Λ, and/or thenumber of layers.

The exemplary fiber illustrated in FIG. 3B, and other holey fibersdisclosed herein, can be formed, for example, by stacking of glasscapillaries to construct a preform. One or more capillaries in thecenter may be replaced by glass rod/rods to form the core. In someembodiments as described below, the core-forming rod(s) can be dopedwith rare earth ions to provide a gain media. The preform can then bedrawn on a conventional fiber-drawing tower into fibers with similarcross-section. Other methods of forming holey fiber may be employed aswell.

In one embodiment, the large core design disclosed herein has asubstantially uniform refractive index distribution, especially over thecore and preferably over the cross-section of the fiber (in the claddingregion, gap areas between the holes). When the core diameter is in therange of approximately 40 to 100 μm (such as those disclosed herein),this uniformity becomes increasingly important to substantiallyeliminate any local waveguide effects within the core of the fiber.

FIGS. 4A-C now illustrate various embodiments of a PCF having claddingregions formed by hexagonally arranged holes or cladding features. Inthe exemplary PCFs shown in FIGS. 4A-C, core dimensions are comparableto hole spacings and hole dimensions. Thus, by selecting a relativelylarge hole size and spacing, a relatively large core dimension can beachieved.

One way to design such a relatively large core is to begin bydetermining the desired dimension of the core. Then, a hexagonallyclose-packed array of holes or cladding features can be arranged wherethe hole diameter d is substantially equal to the hole spacing Λ. Tocreate a core, the center hole (and possibly more holes about the centerhole) can be removed. The resulting “cladding” region comprises one ormore rings or rows of close-packed holes, with the adjacent holessubstantially close or touching. Because such close-packing may reduceleakage of higher order modes, leakage “paths” may be provided andleakage enhanced by reducing the hole dimension, increasing the holespacing, or some combination thereof, such that, for example, theadjacent holes are no longer touching or as close together.

FIG. 4A illustrates one embodiment of a PCF 160 having a cladding region162 formed by one layer or row (N=1) of holes 170. An inner portion 164a of the cladding region 162 generally defines a core 166 having aradius of ρ, and an outer portion 164 b of the cladding region 162 hasan outer diameter R. In the PCF 160 depicted in FIG. 4A, the holediameter d is less than the hole spacing Λ. In one embodiment of an N=1PCF, the hole diameter d is approximately 75 μm, and the hole spacing Λis approximately 80 μm, resulting in the core diameter 2ρ (=2Λ−d) thatis approximately 85 μm.

FIG. 4B illustrates one embodiment of a PCF 180 having a cladding region182 formed by two layers or row (N=2) of holes 170. An inner portion 184a of the cladding region 182 generally defines a core 186 having aradius of ρ, and an outer portion 184 b of the cladding region 182 hasan outer diameter R. In the PCF 180 depicted in FIG. 4B, the holediameter d is less than the hole spacing Λ. In one embodiment of an N=2PCF, the hole diameter d is approximately 35 μm, and the hole spacing Λis approximately 40 μm, resulting in the core diameter 2ρ (=2Λ−d) thatis approximately 45 μm.

FIG. 4C illustrates one embodiment of a PCF 200 having a cladding region202 formed by three layers (N=3) of holes 210. An inner portion 204 a ofthe cladding region 202 generally defines a core 206 having a radius ofρ, and an outer portion 204 b of the cladding region 202 has an outerdiameter R. In the PCF 200 depicted in FIG. 4C, the hole diameter d isless than the hole spacing Λ. In one embodiment of an N=3 PCF, the holediameter d is approximately 15 μm, and the hole spacing Λ isapproximately 20 μm, resulting in the core diameter 2ρ (=2Λ−d) that isapproximately 25 μm.

Table 1, shown below, summarizes various properties of the PCFsdescribed above in reference to FIGS. 4A to 4C, in comparison withvarious conventional PCFs described in Philip Russell: “Photonic crystalfibers,” Science, vol. 299, p. 358-62, 2003, Bouwmans et al.:“High-power Er:Yb fiber laser with very high numerical aperturepump-cladding waveguide,” Applied Physics Letters, vol. 83, p. 817-18,2003, Furusawa et al.: “Cladding pumped Ytterbium-doped fiber laser withholey inner and outer cladding,” Optics Express, vol. 9, p. 714-20,2001, and Wadsworth et al.: “High power air clad photonic crystal fiberlaser,” Optics Express, vol. 11, p. 48-53, 2003. In this table, the“Modes Propagated” is either single-mode (SM) or multi-mode (MM)corresponding to whether only propagation of a single optical mode issupported or whether multiple optical modes are propagated through thefiber, N corresponds to the number of rings or row (layers), d is thehole diameter, Λ is the hole spacing, 2ρ is a measure of the corediameter approximately equal to 2Λ−d for cases where the corecorresponds to a single removed center hole of a hexagonallyclose-packed array, λ is the operating vacuum wavelength, Δn is themodal spacing (both measured and calculated), NA is the numericalaperture (measured and calculated), and V is the normalized frequencyparameter that can be expressed as (2ρ)(NA)π/λ. Reference [1]corresponds to Philip Russell: “Photonic crystal fibers,” Science, vol.299, p. 358-62, 2003; reference [2] corresponds to Bouwmans et al.:“High-power Er:Yb fiber laser with very high numerical aperturepump-cladding waveguide,” Applied Physics Letters, vol. 83, p. 817-18,2003; reference [4] corresponds to Furusawa et al.: “Cladding pumpedYtterbium-doped fiber laser with holey inner and outer cladding,” OpticsExpress, vol. 9, p. 714-20, 2001; and reference [5] corresponds toWadsworth et al.: “High power air clad photonic crystal fiber laser,”Optics Express, vol. 11, p. 48-53, 2003, each of which are herebyincorporated herein by reference in their entirety.

From Table 1, one can see that the conventional single-mode PCFs have arelatively small value of d/Λ, which corresponds to a relatively smallcore size. Furthermore, their cladding region includes a relativelylarge number of rows or layers of holes. Thus, the cladding region inthese conventional PCFs that propagate a single optical mode generallyresembles the cladding (e.g., a glass with slightly lower refractiveindex) of a traditional non-holey single-mode fiber.

Table 1 further shows details of two exemplary large core PCFs thatpropagate only a single optical mode, denoted as “FIG. 4B” and “FIG. 4A”in the reference column. These two PCFs incorporate various designs andtechniques described herein. Some techniques described below were usedto obtain the numerical simulation for these two exemplary PCFs.

The measured index difference Δn_(meas) can be derived from the measurednumerical aperture (NA) of the fiber output. The precision of suchmeasurement can be less than ideal. Since this value is a usefulparameter for comparison, the index difference was calculated for eachcase from the given d and Λ. These calculated values may be moreaccurate than those obtained by direct measurement. TABLE 1 Modes Prop-d Λ d/Λ 2ρ λ Δn_(meas) NA_(meas) V_(meas) Ref. agated N (μm) (μm) (μm)(μm) (μm) (Δn_(cal)) (NA_(cal)) (V_(cal)) [1] SM 5 2.7 9.7 0.3 16.7 1.060.0008 0.048 2.38 (0.00088) (0.0505) (2.50) [2] SM 4 2.65 8.8 0.3 15 10.0042 0.11 5.2 (0.001) (0.054) (2.58) [2] MM 4 5.7 10.4 0.55 15 1 N/A0.056 N/A (0.0021) (0.078) (3.67) [4] SM 6 2 11.5 0.18 28 1.07 0.000860.05 4.1 (0.00042) (0.035) (2.87) [5] MM 3-4 9 12 0.75 17 1.06 0.00170.07 3.5 (0.0051) (0.12) (6.13) FIG. SM 3 15 20 0.75 25 — — — — 4C FIG.SM 2 35 40 0.875 45 1.55 N/A N/A N/A 4B (0.0025) (0.0854) (7.8) FIG. SM1 75 80 0.938 85 1.55 N/A N/A N/A 4A (0.001106) (0.0566) (9.75)

The calculated Δn_(cal) can be obtained by first calculating theequivalent cladding index, which is the effective fundamental mode indexof a structure which has the basic unit cell of the cladding extendinginfinitely in all directions. This calculation was performed with afinite element model with sufficient resolution for each case. The valueof Δn_(cal) is then determined by computing the difference of the glassrefractive index and the equivalent cladding index.

As stated above, 2ρ is a measure of the equivalent core diameter and istaken to be 2Λ−d where a single hole (e.g., removed rod) forms the core.For cases where three absent rods or three holes form the core, thevalue of the core diameter can be obtained, e.g., from Mortensen et al:“Improved large-mode-area endlessly single-mode photonic crystalfibers”, Optics Letters, vol. 28, pp. 393-395, 2003.

As stated above in reference to Table 1, the V parameter is thenormalized frequency parameter (also referred to as V number) that canbe approximated as (2ρ)(NA)π/λ. V_(meas) and V_(cal) correspond tomeasured and calculated V values respectively. In a traditional fiber,V<2.405 corresponds to the region of single mode fiber. As seen in Table1, for all of the PCFs in the top five rows that support only singlemode propagation, the calculate V numbers are less than 2.405(V_(cal)<2.405), or very close to it. In contrast, the calculated Vnumbers for the N=2 (FIG. 4B) and N=1 (FIG. 4A), which are based ondesigns and techniques described herein and where propagation of only asingle optical mode is supported, are 7.8 and 9.75 respectively. Suchlarge V values would be associated with multi-mode fiber for atraditional non-holey optical fiber as well as multi-mode propagation inconventional PCFs.

In all of the single-mode PCFs in the first five rows of Table 1,d/Λ<0.6, whereas the N=2 and N=1 embodiments of PCFs based on designsand techniques described herein have d/Λ>0.6. The relative hole diameterof these PCFs described herein are thus much larger than that ofconventional PCFs. In some embodiments, the fiber comprises a large Λfor large core diameter and λ/Λ<0.3. The hole size din such a region ofinterest may additionally be very large in comparison to wavelength λand to the gaps (glass matrix region, for example) between holes orscatter features.

In one possible region of operation as described above, single modeoperation (where only a single optical mode is supported and higherorder modes are suppressed) can be achieved. High leakage loss isprovided for higher order modes while the fundamental mode leakage lossis kept very low. A PCF can be designed to allow a substantial powerleakage over the device length for the higher order modes, whilemaintaining an acceptably low transmission loss for the fundamentalmode. Some possible ways of optimizing the leakage losses by adjustingthe number of layers of holes (N) and the hole properties are describedbelow in greater detail.

For the large-core single-mode PCFs incorporating the designs andtechniques described herein, a preferred region of operation is d/Λ≧0.35with N≦3. More preferably, d/Λ≧0.4 or d/Λ≧0.6. Even more preferably,d/Λ≧0.75. The core sizes may be between about 20 to 300 micrometers ormore preferably between about 30 to 150 micrometers. In some exemplaryembodiments of holey fiber or PCF, d/Λ>0.6, N≦4, and λ/Λ<0.3. In otherembodiments of holey fiber or PCF, for example, d/Λ>0.75, N≦2, andλ/Λ<0.3. The range of different fiber designs, however, should not belimited to these parameters as wide variation is possible.

FIGS. 5 and 6 now illustrate how, for a given number of layers of holes,leakage loss can be changed by adjusting the hole dimension and holespacing. While N=2 configuration is used for describing FIGS. 5 and 6,it will be understood that the concept applies to other configurationsof N.

FIGS. 5A-C illustrate three exemplary PCFs 240, 250, 260 having asubstantially constant hole spacing Λ. As shown in FIG. 5A, a firstexemplary cladding region 244 comprises two layers of holes 242 having afirst diameter d₁ arranged with the hole spacing Λ. Such a configurationresults in a first core 246 having a first core diameter of 2ρ₁, and afirst gap dimension (between two adjacent holes) of Λ−d₁.

As shown in FIG. 5B, a second exemplary cladding region 254 comprisestwo layers of holes 252 having a second diameter d₂ arranged with thehole spacing Λ. Such a configuration results in a second core 256 havinga second core diameter of 2ρ₂, and a second gap dimension (between twoadjacent holes) of Λ−d₂.

As shown in FIG. 5C, a third exemplary cladding region 264 comprises twolayers of holes 262 having a third diameter d₃ arranged with the holespacing Λ. Such a configuration results in a third core 266 having athird core diameter of 2ρ₃, and a third gap dimension (between twoadjacent holes) of Λ−d₃.

From FIGS. 5A-C, one can see that as the hole diameter decreases(d₁>d₂>d₃), the gap between the holes increases (Λ−d₁<Λ−d₂<Λ−d₃),thereby allowing greater leakage losses. One can also see that the coresize increases (ρ₁<ρ₂<ρ₃) as the hole diameter decreases. Thus, one canselect a desired gap (and possibly its corresponding core size) byselecting a hole size for a given hole spacing.

FIGS. 6A-C now illustrate three exemplary PCFs 270, 280, 290 having asubstantially constant hole dimension d. As shown in FIG. 6A, a firstexemplary cladding region 274 comprises two layers of holes 272 havingthe diameter d arranged with a first hole spacing Λ₁. Such aconfiguration results in a first core 276 having a first core diameterof 2ρ₁, and a first gap dimension (between two adjacent holes) of Λ₁−d.

As shown in FIG. 6B, a second exemplary cladding region 284 comprisestwo layers of holes 282 having the diameter d arranged with a secondhole spacing Λ₂. Such a configuration results in a second core 286having a second core diameter of 2ρ₂, and a second gap dimension(between two adjacent holes) of Λ₂−d.

As shown in FIG. 6C, a third exemplary cladding region 294 comprises twolayers of holes 292 having the diameter d arranged with a third holespacing Λ₃. Such a configuration results in a third core 296 having athird core diameter of 2ρ₃, and a third gap dimension (between twoadjacent holes) of Λ₃−d.

From FIGS. 6A-C, one can see that as the hole spacing increases(Λ₁<Λ₂<Λ₃), the gap between the holes increases also (Λ−d₁<Λ−d₂<Λ−d₃),thereby allowing greater leakage losses. One can also see that the coresize increases (ρ₁<ρ₂<ρ₃) as the hole spacing increases. Thus, one canselect a desired gap (and possibly its corresponding core size) byselecting a hole spacing for a given hole dimension.

The leakage adjustments described above in reference to FIGS. 5 and 6are done by independently adjusting the hole size and hole spacing. Onecan also adjust the leakage by adjusting the hole size and hole spacingin some combination to achieve a desired leakage property. Furthermore,hole size and spacing may change from layer to layer possibly to achievea desired result.

FIGS. 7-9 now illustrate various embodiments of PCFs that incorporaterelatively large cores and are capable of single-mode propagation whereonly a single mode is propagated and propagation of higher order modesare suppressed. FIG. 7 illustrates an exemplary PCF 300 having acladding region 302 that allows single-mode propagation through arelatively large core 304. The PCF 300 further comprises an outercladding 306 disposed on an exterior portion 312 of the PCF 300. Theexemplary outer cladding 306 comprises a plurality of holes 310 arrangedin a hexagonal shape. In one embodiment, the hexagonal shape of theouter cladding 306 is substantially concentric with the hexagonal shapeof the cladding region 302.

Such a “double-clad” fiber can be used, for example, in fiberamplifiers, lasers, CW or pulsed light sources, or in other devices andsystems where the fiber is pumped. An outer cladding formed from holescan provide a number of advantageous features over conventionaldouble-clad fibers. One possible advantage is that a low index polymercoating of conventional double-clad fibers can be eliminated. Low indexpolymer coatings can be damaged relatively easily by heat and highoptical power, thus potentially posing reliability and stabilityproblems for high power applications. Another possible advantage is thata relatively high numerical aperture can be achieved when compared toconventional double-clad fibers. This feature allows for a smaller pumpwaveguide and consequently improved overlap of pump and laser light,possibly leading to a shorter device length. A shorter device, inaddition to being more compact and economical, also reduces nonlineareffects that limit the peak power of lasers and amplifiers.

FIG. 8 illustrates another embodiment of a PCF 320 having a claddingregion 322 that allows single-mode propagation through a relativelylarge core area. The PCF 320 further comprises a doped core 324 disposedin the core area. Such a doped core PCF may include an outer claddingformed by a plurality of holes disposed on an outer region 326 about thecladding region 322. In one embodiment, the doped core 324 comprises arare earth doped core that allows the PCF 320 to be used as a gainmedium such as in a fiber laser or a fiber amplifier.

FIGS. 9A-D illustrate various embodiments of PCFs having claddingregions that allow single-mode propagation through relatively large coreareas. The cladding regions include asymmetries that facilitatepolarization maintenance of single-mode polarized optical signalspropagating through the respective cores.

FIG. 9A illustrates an exemplary PCF 330 having a cladding region 332that allows single-mode propagation through a relatively large corearea. In the exemplary PCF 330, the core area is occupied with a dopedcore 340 to provide a gain medium. The cladding region 332 comprises aplurality of first holes 334 that form one or more layers. The claddingregion 332 includes an asymmetry introduced by second holes 336 that aredifferent from the first holes 334. In one embodiment, the second holes336 are smaller than the first holes 334, and are aligned on oppositesides of the core area thereby allowing a preferential orientation ofthe optical signal's electric field.

FIG. 9B illustrates an exemplary PCF 350 having the cladding region 332(similar to FIG. 9A) that allows single-mode propagation through arelatively large core 360. The cladding region 332 and the asymmetrytherein allows polarization maintained single-mode propagation throughthe core 360.

FIG. 9C illustrates an exemplary PCF 370 having a cladding region 372that allows single-mode propagation through a relatively large corearea. Similar to the PCF 330 of FIG. 9A, the exemplary PCF 370 has adoped core 380 to provide a gain medium. An asymmetry of the claddingregion 372 is achieved by smaller holes 376 (smaller than holes 374)disposed about the core 380, thereby allowing a preferential orientationof the optical signal's electric field.

FIG. 9D illustrates an exemplary PCF 390 having the cladding region 372(similar to FIG. 9C) that allows single-mode propagation through arelatively large core 400. The cladding region 372 and the asymmetrytherein allows polarization maintained single-mode propagation throughthe core 400.

As shown in FIGS. 9E and 9F, the position of the first larger set ofholes 334 and the position of the second smaller set of holes 336 can beselected such that the separation (s₁) of the larger holes 334 isapproximately equal to the separation (s₂) of the smaller holes 336(s₁≈s₂). (These separations s₁, s₂ and general position of the holes334, 336 may be measured, for example, from the center of the holes 334,336.) This configuration provides a more circular fundamental mode,which is desirable in many applications. In various preferredembodiments, the positions of these holes 334, 336 are located atlattice points corresponding to the lattice (e.g., a generally hexagonallattice) having an average spacing Λ, as described above. Alternatively,the small holes 336 can be closer (see holes in phantom in FIG. 9E) orthe large holes 334 (see hole in phantom in FIG. 9F) can be fartherapart. FIG. 9E shows the small holes 336 shifted with respect to thelattice positions and in particular toward the center of the fiber. Bycontrast, FIG. 9F shows the large holes 334 shifted with respect to thelattice positions and in particular away the center of the fiber. Otherconfigurations are also possible.

The various PCFs described above in reference to FIGS. 2-9 have coresthat occupy areas where the respective center holes would otherwise belocated. It will be appreciated that a relatively large core may includearea(s) in other regions as well. As an example, two adjacent holes ofthe first layer may be removed to yield a triangular shaped core that isslightly off-center from the geometric center of the hexagonal shapedcladding region. In another example, the whole first layer may beremoved so as to yield a larger core whose dimension is similar to thatof the outer periphery of the first layer.

Whether such enlarged cores can support single-mode propagation dependson the extent of leakage losses of non-fundamental (higher order) modeswith respect to the loss of the fundamental mode for a given length ofthe PCF. Various properties of the holey cladding described above (holesize, hole spacing, number of layers, hole composition etc.) may beadjusted to achieve single-mode propagation through the enlarged core.Alternatively, various properties of the holey cladding may be selectedto allow propagation of a few selected modes of an optical signal.

As used herein, the “optical signal” refers broadly to propagatedoptical energy or power and is not limited to an optical carrier forvoice, data, or information. For example, the optical signal maycorrespond to the light propagating through an optical fiber amplifieror the output of a fiber optic laser or other light source.

One aspect of the present teachings relates to propagating a single-modesignal through a fiber that supports multiple modes. Because the“mode-selection” in holey fibers depends on the relative leakage lossesof different modes, the holey fiber can be configured to allowpropagation of a several non-fundamental modes configuring the claddingregion to “include” (i.e., not allow substantial loss by leakage) thesehigher order modes. Such a holey fiber for propagating multiple modes(e.g., about 20 modes or more) or “few”-modes can be made, for example,by removing one or more inner layers, as illustrated in FIGS. 10A and B.

In FIG. 10A, an exemplary PCF 410 comprises a cladding region 412 formedby a hexagonal array of holes 414. In one embodiment, a core 420 isformed in an area where the center hole and the first layer of holes(not shown) would otherwise be located in the array. The exemplary PCF410 may further include an outer cladding disposed on an outer region416.

In FIG. 10B, an exemplary PCF 430 comprises a cladding region 432 formedby a hexagonal array of holes 434. In one embodiment, a core 440 isformed by additionally removing the second layer of holes. The exemplaryPCF 430 may further includes an outer cladding disposed on an outerregion 416. Thus, one can see that the resulting core 440 (and the core420 of FIG. 10A) can be relatively large to allow handling of high poweroptical signals without excess intensity levels being reached that causeoptical non-linear effects, which might impair operation.

FIGS. 10C and D now illustrate how a single-mode signal can bepropagated through a given length of a PCF that support a few opticalmodes. In FIG. 10C, one embodiment of a PCF 450 that supports “few”-modecomprises two (exemplary) layers 454 a, 454 b of holes forming acladding region 452 about a relatively large core 460. Although an outercladding 456 is shown in FIG. 10C, such a feature may not be desired insome embodiments.

As shown in FIG. 10C, a single-mode signal 464 without higher ordermodes is launched into the core 460 of the PCF 450 from a signal source462. The single-mode signal 464 is shown to substantially survive thelength L of the PCF 450 to emerge as a single-mode signal 466. Thesurvival of the single mode signal can be facilitated by a relativelylarge modal spacing of the PCF. PCFs that support multiple optical modescan be designed to have large modal spacing, greater than approximately0.001, between the fundamental mode and the next higher order mode whilehaving a large core (with a diameter potentially in the hundreds ofmicrons). The large modal spacing reduces inter-modal coupling. Thus, afundamental mode signal (464 in FIG. 10C) launched into the PCF thatsupport propagation of multiple modes is expected to be robustly guidedin that single mode through a given length of the PCF.

A variety of techniques may be employed to launch a single-mode into thefiber that supports multiple optical modes. For example, an opticalcoupler may be disposed with respect to the core region to couple lightinto the core region. The optical coupler may be configured to outputlight having an optical distribution substantially matching the modeprofile of the fundamental mode. More of the light is therebysubstantially coupled into the signal mode of the fiber than higherorder modes. The optical coupler may comprises, for example, a lens orother optics having suitably characteristics (e.g., optical power,numerical aperture, etc.) and that is disposed with respect to the coreregion to focus light down to a size substantially matching the size ofthe mode profile of the fundamental mode. Alternatively, the opticalcoupler may comprise a waveguide that outputs light having adistribution substantially matching the mode profile of the fundamentalmode. Other techniques may be employed as well.

FIG. 10D illustrates a block diagram of an exemplary optical signalpropagating system 470 that reduces a high power multi-mode signal 472,with, e.g., 20 or more optical modes, to a single-mode signal 486. Oneor more segments of a PCF that supports a “few” optical modes asdescribed above in reference to FIGS. 10A-C are employed. The exemplarysignal 472 having many modes is shown to be launched into a first PCF474 having a first length L₁ that supports a few optical modes. Aftertraveling through the first length L₁, the many-mode signal 472 isreduced to a few-mode signal 476 because modes other than the few modesare lost by leakage. The “few”-mode signal 476 is then shown to passthrough a filter 480 that filters out non-fundamental modes from thefew-mode signal 476. The filter 480 can be configured to substantiallyeliminate the power in the higher order mode(s) while allowing passageof the fundamental mode. Such mode filtering can be implemented bydevices and/or techniques such as a tapers, bends, grating such as longperiot grating to couple higher order modes into leaky modes, andgain/loss profiling. Mode filters are described in U.S. Pat. No.5,818,630 issued to Fermann et al on Oct. 6, 1998 and entitled“Single-mode Amplifier and Compressors Based on Multi-mode Fibers”(Attorney Docket IM-59), which is hereby incorporated herein byreference.

As further shown in FIG. 10D, a single-mode signal 482 emerges from thefilter 480. The single-mode signal 482 can then be guided through asecond PCF 484 (length L₂) that supports few-mode propagation to resultin the output single-mode signal 486.

Another aspect of the present teachings relates to a PCF having aplurality of holes arranged in an array or lattice that meets the Braggcondition for a given signal wavelength. In accordance with descriptionpresented above, such a PCF is referred to as a PBF (photonic bandgapfiber). FIG. 11 illustrates an exemplary PBF 490 having an array 492 ofholes 494 disposed on a fiber material 476. The array 492 defines a corearea 500 through which the signal propagates.

Because a PBF relies on Bragg reflection or scattering rather thanaverage refractive index difference, the core can have a low refractiveindex. Consequently, the core can be evacuated or filled with air orother gases or medium that have very low nonlinear effects. Such anadvantageous feature can lead to a significant reduction of nonlineareffect, and consequently, very high optical power can propagate in thesePBFs without suffering from nonlinear distortion.

In one embodiment, the core 500 further comprises a doped ring-shaped orannular region 502 disposed about the center of the core. As FIG. 11 isa cross-sectional view of the fiber, the doped region 502 may comprise adoped portion having a shape of a hollow right circular cylinder.Accordingly, an open inner region 504 of the doped ring 502 can befilled with air or other gases to reduce the nonlinear effect of thePBF. Various other configurations are possible. For example, the dopedregion may be rectilinear and have a square or rectangular cross-sectionwhen viewed from the vantage point presented in FIG. 11. Alternatively,the cross-sectional view of the doped region may be triangular,pentagonal, hexagonal, octagonal, or have any wide variety of regular orirregular shapes. This region also need not be contiguous and maycomprise lobes or segmented portions. For example, the doped region maycomprise two or more portions oppositely situated with respect to thehollow core. Still other variations are possible. In variousembodiments, however, the doped region 502 in the PBF core can providegain for lasers and amplifiers; and such lasers and amplifiers can havevery high outputs.

FIGS. 12-14 illustrate various properties of holey fibers and results ofcomputer simulations that characterize the fundamental and first-ordermodes. FIG. 12 illustrates a loss profile 510 for the fundamental andfirst-order modes of a PCF as a function of the number of layers ofholes. For this simulation, hole dimension and hole spacing were d=35 μmand L=40 μm respectively (thus having a core diameter of approximately45 μm). The N=2 data point corresponds to the cladding regionconfiguration described above in reference to FIG. 4B, and the N=1 datapoint corresponds to the cladding region with the second layer removed.

One can see that the leakage loss depends strongly on the number oflayers. As expected, a reduction in N (N=2 to N=1) results in anincrease in power leakage for both modes. For the fundamental mode, thereduction in N (2 to 1) causes a leakage increase by more than threeorders of magnitude. For the first-order mode, the same reduction in Ncauses a leakage increase by about two orders of magnitude.

Because of such a strong dependence in N, one approach to designing aPCF for a particular application is to first coarsely tune the leakageloss with selection of the number of layer or row, N. Once the leakageloss is in the desired general or broad range, fine-tuning of theleakage loss can be performed by varying the hole size. As expected, andas described above in reference to FIGS. 5A-C, smaller holes generallylead to higher power leakage.

In general for higher-modes, increases in power leakage aresubstantially greater for different value of N, as well as for smallerholes. Thus to design a PCF that propagates only a single optical mode,selecting the value of N and the hole size to substantially eliminatethe first-order mode over a given length ensures that higher modes arealso substantially eliminated.

In certain applications of PCFs, such as fiber lasers, amplifiers anddelivery fibers, the fiber length is on the order of few meters or less.In such applications, N≦3 can achieve a single-mode operation ford/Λ≧0.6. Preferably, d/Λ≧0.75.

FIG. 13 illustrates a loss profile 520 as a function of bending of anexemplary PCF having two layers of air-filled holes, with d=35 μm andΛ=40 μm (thus with a core diameter of approximately 45 μm). Thefundamental mode is largely confined by the air/hole boundary. Thisproperty leads to usefully low bending loss, arising from the fact thatthe mode in the region of the bend is substantially reduced as a resultof the strong confinement of the air-hole boundary. At very smallbending diameters, i.e. about 25 mm (inverse bending radius ofapproximately 0.04 mm⁻¹), fundamental mode loss on a bend can be largewhile first order mode loss on a bend can be substantially reduced bythe mode reduction effect described below.

The bending loss is usually referred to as propagation loss associatedwith propagation through bends. To obtain the results shown in FIG. 13,the mode transition loss from straight fiber modes to bent fiber modewas not accounted for in the model. Such transitional loss depends onthe nature of the transition and can be negligible on a slow adiabatictransition.

One can see that the amount of bend of a PCF can be used to determinehow much power in the fundamental and first-order modes are lost due tobending. To aid in designing for bend induced loss, some of theproperties of bend loss in PCFs described below can be useful.

The bending sensitivity of these types of fibers (e.g. large-hole PCFs)is very low. This result is due to the fact that modal area isprogressively reduced with the decreasing diameter of the bends, whicharises from the fact that the modal field is restricted from penetratinginto the large nearby air holes. This characteristic is significantlydifferent from traditional fibers and conventional PCFs, where modalfield is displaced from the center with much less level of fielddistortion on a bend. As a consequence, the modal field moves beyond thecore-clad boundary and produces large bending loss in traditionalnon-holey fibers and convention PCFs. This effect is especially true forlarge core designs in traditional fibers and conventional PCFs.

FIG. 14 illustrates how the effective modal indices of the fundamentaland first-order modes for the exemplary large-core PCF of FIG. 13 (withtwo layers of holes, N=2, d=35 μm and Λ=40 μm, core diameter of 45 μm)changes as a function of inverse bending radius. The difference betweenthe fundamental mode curve and the first-order mode curve corresponds tothe modal spacing between the fundamental and first-order modes. One cansee that the modal spacing is greater than approximately 0.005throughout the measured range of bends. Such modal spacing (0.005) ismuch larger than that of conventional PCFs and traditional fibers. Thehigh modal spacing is indicative of a very low inter-modal coupling, andis also an indicator of a robust single-mode operation. As a result ofthe high modal spacing (about 0.005), a perturbation of sub-millimeterperiod is needed for sufficient phase-matching to allow substantialinter-modal coupling, for example, in the exemplary N=2 PCF.

High modal spacing extends to even larger core diameters. For fiberswhose core diameter is on the order of about 100 μm (such as the N=1large core PCF of FIG. 4A), the fundamental to next higher order modespacing is still significantly larger than that of traditional non-holeyfibers and conventional PCFs. Again, this feature produces relativelylow inter-modal coupling.

When core size is increased further, the modal spacing generallydecreases. This trend, together with increased bending loss, mayeventually set an upper limit on the maximum core diameter.Nevertheless, an increase in the core diameter by an order of magnitudeover the largest conventional PCF that supports single-mode propagation(see, e.g., reference [4] in Table 1, with core diameter of 28 μm) maybe possible with the techniques and designs described herein.

FIGS. 15-16 illustrate a method of characterizing the various PCFs forpropagating only a single mode that also have large cores. FIG. 15illustrates an exemplary pair of holes or cladding features 550, showingthe hole's diameter d and the hole spacing Λ. Such holes arranged in ahexagonal manner as described above can lead to the variousrelationships between λ/Λ and d/Λ as illustrated in FIGS. 16A-C. Otherconfigurations are also possible.

FIG. 16A illustrates a curve 562 that separates a single-mode (SM)propagation regime where only a single optical mode is propagated from amulti-mode (MM) propagation regime where multiple optical modes arepropagated on a plot of λ/Λ versus d/Λ for PCFs. Conventional PCFs thatoperate in the SM propagation regime have a relatively small core size,as well as a relatively large number of hole layers (N≧4), such that theoverall cross-sectional profile is generally similar to that oftraditional fibers. For such a fiber, the value of d/Λ is approximately0.35 in the limit when λ/Λ approaches zero. Thus, a region to the rightof the curve 562 in FIG. 16A (i.e., increasing hole size relative tospacing, d/Λ) where d/Λ≧0.4 causes the PCF to become a MM fiber, therebylimiting the operable core size for SM operation.

FIG. 16B illustrates a curve 582 for the N=2 embodiment described abovein reference to FIG. 4B and Table 1. In that embodiment, the value of(d/Λ)₂ is approximately 0.88 when λ/Λ equals about 0.1 (for 10 meters offiber, while this value would be larger for longer fiber). Thus, one cansee that a usable SM propagation regime is advantageously extended fromthe curve 562 to the curve 582 as indicated by an arrow 584. A largerhole size relative to spacing can be used while maintaining SMoperation. By choosing a large hole size, a corresponding hole spacingcan yield a similarly large core size, as seen above in reference toTable 1.

FIG. 16C illustrates a curve 592 for the N=1 embodiment described abovein reference to FIG. 4A and Table 1. In that embodiment, the value of(d/Λ)₁ is approximately 0.94 when λ/Λ equals about 0.1 (for 10 meters offiber, while this value would be larger for longer fiber). Thus, one cansee that a usable SM propagation regime is advantageously extended evenmore from the curve 562 to the curve 592 as indicated by an arrow 594thereby providing a larger core size for SM operation.

FIGS. 17-18 illustrate a generalized concept of the SM propagationregime extension described above in FIGS. 15-16 in context of holeyfibers. As shown in FIG. 17, and described above in reference to FIGS.1A and B, a holey fiber having the advantageously large core operable inSM can have an at least partially irregular or disordered arrangement ofholes. Furthermore, the holes do not necessarily have to be of uniformsize or shape. Such non-uniformity of holes is depicted in FIG. 17,where two exemplary holes 600 and 602 are circular and square shapes,respectively. Furthermore, the circular hole 600 is shown to have adimension of d_(i), and the square hole 602 is shown to have a dimensionof d_(j). An exemplary spacing between the two exemplary holes 600 and602 is denoted as Λ_(k).

For holey fibers whose cladding region is formed by such non-uniformholes, the hole spacing parameter and hole dimension parameter can beexpressed as average values d_(avg) and Λ_(avg). Using such averageparameters, a similar λ/Λ versus d/Λ relationship (as that of FIG. 16)can be expressed. FIG. 18A illustrates a curve 612 that separates the SMpropagation regime from the MM propagation regime in theλ/Λ_(avg)−d_(avg)/Λ_(avg) space. The value (d_(avg)/Λ_(avg))_(min)represents the value of d_(avg)/Λ_(avg) in the limit when λ/Λ_(avg)approaches zero. Similar to the threshold value of d/Λ beingapproximately 0.35 for conventional PCFs, the (d_(avg)/Λ_(avg))_(min)can represent a threshold value for conventional holey fibers having acladding region having a relatively thick average thickness about a corehaving a relatively small average dimension.

FIG. 18B illustrates a concept of extending the SM propagation regime ina manner similar to that described above in reference to FIGS. 16B-C. Ausable SM propagation regime is advantageously extended from the curve612 to a curve 622 as indicated by an arrow 624 thereby providing alarger average core size for SM operation. By choosing a large averagehole size, a corresponding average hole spacing can yield a similarlylarge average core size.

The various fibers disclosed herein may be used, for example, in gainfiber, amplifiers, and lasers such as fiber amplifiers and fiber lasersand other systems as well. For example, these fibers may be employed toconstruct high power pulsed and CW laser systems or light sources. Thevarious fibers disclosed herein may be used to construct high powerpulsed and CW laser systems or light sources. These systems may comprisepumps, a length of fiber to provide gain media and a pair of mirrors attwo ends of a fiber to form a cavity. Polarizing components can be addedto achieve single polarization mode operation where only a singlepolarization mode is propagated. Dispersing elements can also be addedfor intra-cavity dispersion control. The fibers disclosed herein canalso form the gain media in pulsed and CW optical amplifiers, whichcomprise of one or more optical pumps, pump couplers, and possiblyisolators. The fibers disclosed herein can be used to form part of afiber delivery system, where the output of a laser is launched into thefiber through a focusing element or a splice while the output of thefiber is directed by additional optics to a target. Other configurationsand uses are possible.

The fibers disclosed herein can also be utilized in telecom applicationsto replace MM telecom fibers for short haul data transmission, where alarge core size enables low cost and robust connectors while the singlemode nature of the fiber can improve the bandwidth of the transmissionsystem. In such applications, a second glass can be used instead ofholes. Still other applications are possible.

Although the above-disclosed embodiments of the present invention haveshown, described, and pointed out the fundamental novel features of theinvention as applied to the above-disclosed embodiments, it should beunderstood that various omissions, substitutions, and changes in theform of the detail of the devices, systems, and/or methods illustratedmay be made by those skilled in the art without departing from the scopeof the present invention. Consequently, the scope of the inventionshould not be limited to the foregoing description, but should bedefined by the appended claims.

All publications and patent applications mentioned in this specificationare indicative of the level of skill of those skilled in the art towhich this invention pertains. All publications and patent applicationsare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

1. A hollow core photonic band gap fiber that provides optical amplification, said hollow core photonic band gap fiber comprising: a cladding region comprising a plurality of cladding features disposed in a matrix; a hollow core region surrounded by said cladding region, said hollow core having a hollow region formed therein, said plurality of cladding features arranged to form a photonic band gap to substantially confine propagation of light in said hollow core region; and a gain region having optical gain that is disposed proximal to said hollow region such that said light propagating through said hollow core region is amplified by said gain provided by said gain region.
 2. The hollow core photonic band gap fiber of claim 1, wherein said gain region is doped with a material that provides gain.
 3. The hollow core photonic band gap fiber of claim 1, wherein said gain region comprises an annular region surrounding said hollow core.
 4. An optical fiber amplifier comprising: an optical gain fiber for propagating a single mode, said optical gain fiber providing optical gain, said optical gain fiber comprising: (i) a cladding region comprising a plurality of cladding features disposed in a matrix, said plurality of cladding features having an average spacing, Λ, and an average size, d; and (ii) a core region surrounded by said cladding region, said plurality of cladding features substantially confining propagation of said single mode to said core region, said plurality of cladding features having (i) sufficient average spacing, Λ, to provide an increased effective core size, 2ρ, and (ii) a sufficient average size, d, to provide substantial confinement of light within said core region, wherein the difference between said average size, d, and said average spacing, Λ, is less than about 50% of said average spacing, Λ.
 5. An optical fiber amplifier comprising: an optical pump source; and an optical gain fiber optical fiber for propagating a single optical mode, said optical gain fiber providing optical gain, said optical gain fiber comprising: (i) a cladding region comprising a plurality of cladding features disposed in a matrix, said plurality of cladding features having an average spacing, Λ, and an average size, d; and (ii) a core region surrounded by said cladding region, said plurality of cladding features substantially confining propagation of said single optical mode to said core region, said plurality of cladding features having (i) sufficient average spacing, Λ, to provide an increased effective core size, 2ρ, and (ii) a sufficient average size, d, to provide substantial confinement of light within said core region, wherein said average size, d, is substantially equal to said average spacing, Λ.
 6. A CW fiber laser comprising: a resonant cavity formed by at least partially reflective surfaces; and an optical fiber in said resonant cavity, said optical fiber having optical gain, said optical fiber propagating a single optical mode, said optical fiber comprising: (i) a cladding region comprising a plurality of cladding features disposed in a matrix, said plurality of cladding features having an average spacing, Λ, and an average size, d; and (ii) a core region surrounded by said cladding region, said plurality of cladding features substantially confining propagation of said single optical mode to said core region, said plurality of cladding features having (i) sufficient average spacing, Λ, to provide an increased effective core size, 2ρ, and (ii) a sufficient average size, d, to provide substantial confinement of light within said core region, wherein said average size, d, and said average spacing, Λ, have values at least about 20 micrometers to provide an effective core size of at least about 20 micrometers. 